You can buy 1/4" thick fiberboard (or Masonite) at Home Depot and make your own backboard, which is what I did on my MJ, as in, behind the front seat. It does not bend well. You'll have a hard time finding NEW headliner backing for an XJ anywhere other than a junkyard.

A quick google search didn't turn up any options for me, so i'm betting paradise is dead on - you're going to have to hit up some junkyards. Alternatively, you may have an auto-upholestry shop nearby that's capable of making you a whole new headliner for the right price.... if you're just going to run offroad, have you already pulled the headliner out? you may find you don't dislike the jeep without it so much...

If your up for it, and the liner you have is kind of in one piece- try brushing a bunch of fiberglass resin on it. It soaks in good and is pretty strong. You could probably figure out how to patch some missing pieces too. The just use gel contact cement for the fabric. Good luck!!